Kinetics and Mechanisms of the Gold-Catalyzed Hydroamination of Axially Chiral 1‑Aryl-1,2-butadienes with Aniline

The mechanisms of the hydroamination of enantiomerically enriched, axially chiral (R)-1-aryl-1,2-butadienes (R)-(4-C6H4R)­C­(H)=C=C­(H)­Me [R = Ph (1a), OMe (1b)] with aniline catalyzed by (IPr)­AuOTf have been investigated employing a combination of kinetic and stereochemical analyses and in situ spectroscopy of catalytically active mixtures. These experiments were consistent with mechanisms involving essentially irreversible displacement of aniline from [(IPr)­Au­(NH2Ph)]+ by allene (R)-1 to form the enantiomerically enriched gold−π-allene intermediate {(R)-(IPr)­Au­[η2-(R)-ArC­(H)=C=C­(H)­Me]}+ [(R)-I], followed by unimolecular stereomutation to form the racemic gold−π-allene intermediate rac-I. Intermediate rac-I, in turn, undergoes either allene displacement to form racemic allene rac-1 or C–N bond formation/protodeauration to form racemic (E)-N-(4-aryl-3-buten-2-yl)­aniline (rac-2), favoring the former pathway by a factor of ∼2.4. As such, approximately 30% of the racemic product rac-2 is formed directly from (R)-1 without the intermediacy of rac-1.